Acyclic overlays used for broker-based publish/subscribe systems provide unique paths for content-based routing from a publisher to interested subscribers. Cyclic overlays may provide multiple paths, however, the subscription broadcast process generates one content-based routing path per subscription. This poses serious challenges in offering dynamic routing of notifications when congestion is detected because instantaneous updates in routing tables are required to generate alternative routing paths. This paper introduces the first subscription-based publish/subscribe system, OctopiS, which offers inter-cluster dynamic routing when congestion in the output queues is detected. OctopiS is based on a formally defined Structured Cyclic Overlay Topology (SCOT). SCOT is divided into homogeneous clusters where each cluster has equal number of brokers and connects to other clusters through multiple inter-cluster overlay links. These links are used to provide parallel routing paths between publishers and subscribers connected to brokers in different clusters. While aiming at deployment at data center networks, OctopiS generates subscription-trees of shortest lengths used by Static Notification Routing (SNR) algorithm. Dynamic Notification Routing (DNR) algorithm uses a bit-vector mechanism to exploit the structuredness of a clustered SCOT to offer inter-cluster dynamic routing without making updates in routing tables and minimizing load on overwhelmed brokers and congested links. Experiments on a cluster testbed with real world data show that OctopiS is scalable and reduces the number of inter-broker messages in subscription delivery by 89%, subscription delay by 77%, end-to-end notification delay in static and dynamic routing by 47% and 58% respectively, and the lengths of output queues of brokers in dynamic routing paths by 59%.